diff --git a/.github/workflows/python-test.yml b/.github/workflows/python-test.yml
index faf0da2..0475309 100644
--- a/.github/workflows/python-test.yml
+++ b/.github/workflows/python-test.yml
@@ -18,4 +18,4 @@ jobs:
           python-version-file: ".python-version"
 
       - name: Run Tests
-        run: make uv-test
+        run: make uv-test-isolated
diff --git a/Makefile b/Makefile
index f931e9d..035bbf5 100644
--- a/Makefile
+++ b/Makefile
@@ -3,6 +3,7 @@ DOCKER_COMPOSE := docker compose -f docker-compose.yml
 DOCKER_COMPOSE_DEV := $(DOCKER_COMPOSE) -f docker-compose.local.yml
 PYTEST := $(UV) run pytest
 RUFF := $(UV) run ruff
+TEST_PKG := ./scripts/test-package.py
 
 # Aggregate check: can be run in parallel with -j
 check: lint test format-check lock-check
@@ -21,8 +22,31 @@ lock-check: uv-lock-check
 uv-lint: uv-dev
 	$(RUFF) check
 
+# All workspace packages with tests
+TEST_PACKAGES := stitch-api stitch-models
+
+define newline
+
+
+endef
+
+# --- local: full sync, no --exact (fast, no venv mutation) ---
+ifdef pkg
+uv-test: uv-dev
+	$(TEST_PKG) $(pkg)
+else
 uv-test: uv-dev
-	$(PYTEST) deployments/api
+	$(foreach p,$(TEST_PACKAGES),$(TEST_PKG) $(p)$(newline))
+endif
+
+# --- isolated (CI): per-package --exact deps only ---
+ifdef pkg
+uv-test-isolated:
+	$(TEST_PKG) --exact $(pkg)
+else
+uv-test-isolated:
+	$(foreach p,$(TEST_PACKAGES),$(TEST_PKG) --exact $(p)$(newline))
+endif
 
 uv-format: uv-dev
 	$(RUFF) format
@@ -127,7 +151,7 @@ prod-docker:
 .PHONY: all build clean \
         build-python \
         check lint test format format-check \
-        uv-lint uv-test uv-format uv-format-check \
+        uv-lint uv-test uv-test-isolated uv-format uv-format-check \
         uv-sync uv-sync-dev uv-sync-all \
         uv-dev \
         clean-build clean-cache \
diff --git a/packages/stitch-models/tests/conftest.py b/packages/stitch-models/tests/conftest.py
index 4087288..530ed11 100644
--- a/packages/stitch-models/tests/conftest.py
+++ b/packages/stitch-models/tests/conftest.py
@@ -3,8 +3,9 @@
 from __future__ import annotations
 
 from typing import Literal, Mapping
-from uuid import UUID
+from uuid import UUID, uuid4
 
+from pydantic import Field
 import pytest
 
 from stitch.models import (
@@ -24,7 +25,7 @@ class FooSource(Source[int, Literal["foo"]]):
     value: float
 
 
-class BarSource(Source[str, Literal["bar"]]):
+class BarSource(Source[UUID, Literal["bar"]]):
     source: Literal["bar"] = "bar"
     label: str
 
@@ -39,16 +40,16 @@ class UuidSource(Source[UUID, Literal["uuid_src"]]):
 
 
 class FooPayload(SourcePayload):
-    foos: Mapping[int, FooSource] = {}
+    foos: Mapping[int, FooSource] = Field(default_factory=dict)
 
 
 class MultiPayload(SourcePayload):
-    foos: Mapping[int, FooSource] = {}
-    bars: Mapping[str, BarSource] = {}
+    foos: Mapping[int, FooSource] = Field(default_factory=dict)
+    bars: Mapping[str, BarSource] = Field(default_factory=dict)
 
 
 class UuidPayload(SourcePayload):
-    uuids: Mapping[UUID, UuidSource] = {}
+    uuids: Mapping[UUID, UuidSource] = Field(default_factory=dict)
 
 
 # ---------------------------------------------------------------------------
@@ -60,15 +61,15 @@ class EmptyPayload(SourcePayload):
     pass
 
 
-class FooResource(Resource[int, int, str, FooPayload]):
+class FooResource(Resource[int, FooPayload]):
     pass
 
 
-class MultiResource(Resource[int, int, str, MultiPayload]):
+class MultiResource(Resource[int, MultiPayload]):
     pass
 
 
-class ExtendedResource(Resource[int, int, str, EmptyPayload]):
+class ExtendedResource(Resource[int, EmptyPayload]):
     extra: str
 
 
@@ -85,7 +86,7 @@ def __init__(self, id: int, source: str, value: float):
 
 
 class BarSourceORM:
-    def __init__(self, id: str, source: str, label: str):
+    def __init__(self, id: UUID, source: str, label: str):
         self.id = id
         self.source = source
         self.label = label
@@ -103,7 +104,7 @@ def foo_source():
 
 @pytest.fixture
 def bar_source():
-    return BarSource(id="abc", label="test")
+    return BarSource(id=uuid4(), label="test")
 
 
 @pytest.fixture
diff --git a/packages/stitch-models/tests/test_resource.py b/packages/stitch-models/tests/test_resource.py
index 5bdfaa0..6c3d1fd 100644
--- a/packages/stitch-models/tests/test_resource.py
+++ b/packages/stitch-models/tests/test_resource.py
@@ -3,7 +3,7 @@
 from __future__ import annotations
 
 import json
-from uuid import UUID
+from uuid import UUID, uuid4
 
 import pytest
 from pydantic import ValidationError
@@ -112,7 +112,7 @@ def test_rejects_missing_source_data(self):
         assert "source_data" in missing_locs
 
     def test_rejects_wrong_source_in_payload(self):
-        bar = BarSource(id="abc", label="test")
+        bar = BarSource(id=uuid4(), label="test")
         with pytest.raises(ValidationError) as exc_info:
             FooPayload(foos={1: bar})  # type: ignore[arg-type]
         errors = exc_info.value.errors()
diff --git a/packages/stitch-models/tests/test_source.py b/packages/stitch-models/tests/test_source.py
index 3aa1e21..d6aaae0 100644
--- a/packages/stitch-models/tests/test_source.py
+++ b/packages/stitch-models/tests/test_source.py
@@ -3,7 +3,7 @@
 from __future__ import annotations
 
 import json
-from uuid import UUID
+from uuid import UUID, uuid4
 
 import pytest
 from pydantic import ValidationError
@@ -42,7 +42,7 @@ def test_source_default_from_subclass(self):
         foo = FooSource(id=1, value=1.0)
         assert foo.source == "foo"
 
-        bar = BarSource(id="abc", label="test")
+        bar = BarSource(id=uuid4(), label="test")
         assert bar.source == "bar"
 
     def test_id_type_specialization(self):
@@ -51,10 +51,11 @@ def test_id_type_specialization(self):
         assert foo.id == 42
         assert isinstance(foo.id, int)
 
-        # str id
-        bar = BarSource(id="abc", label="test")
-        assert bar.id == "abc"
-        assert isinstance(bar.id, str)
+        # UUID id
+        uid = uuid4()
+        bar = BarSource(id=uid, label="test")
+        assert bar.id == uid
+        assert isinstance(bar.id, UUID)
 
         # UUID id
         uid = UUID("550e8400-e29b-41d4-a716-446655440000")
@@ -77,9 +78,10 @@ def test_from_attributes_orm_object(self):
         assert foo.source == "foo"
         assert foo.value == 3.14
 
-        bar_orm = BarSourceORM(id="abc", source="bar", label="test")
+        uid = uuid4()
+        bar_orm = BarSourceORM(id=uid, source="bar", label="test")
         bar = BarSource.model_validate(bar_orm, from_attributes=True)
-        assert bar.id == "abc"
+        assert bar.id == uid
         assert bar.label == "test"
 
 
@@ -106,11 +108,11 @@ def test_int_id_rejects_non_numeric_string(self):
             FooSource.model_validate_json(payload)
         assert_has_error(exc_info.value.errors(), type="int_parsing", loc=("id",))
 
-    def test_str_id_rejects_int_via_json(self):
+    def test_uuid_id_rejects_int_via_json(self):
         payload = json.dumps({"id": 123, "source": "bar", "label": "test"})
         with pytest.raises(ValidationError) as exc_info:
             BarSource.model_validate_json(payload)
-        assert_has_error(exc_info.value.errors(), type="string_type", loc=("id",))
+        assert_has_error(exc_info.value.errors(), type="uuid_type", loc=("id",))
 
     def test_uuid_id_rejects_malformed_string(self):
         payload = json.dumps({"id": "not-a-uuid", "source": "uuid_src"})
diff --git a/scripts/test-package.py b/scripts/test-package.py
new file mode 100755
index 0000000..0dd1f66
--- /dev/null
+++ b/scripts/test-package.py
@@ -0,0 +1,45 @@
+#!/usr/bin/env -S uv run --script
+# /// script
+# requires-python = ">=3.12"
+# ///
+
+import argparse
+import subprocess
+import sys
+import tomllib
+from pathlib import Path
+
+
+def find_package_dir(name: str) -> str:
+    root = tomllib.loads(Path("pyproject.toml").read_text())
+    for member in root["tool"]["uv"]["workspace"]["members"]:
+        p = Path(member) / "pyproject.toml"
+        if p.exists() and tomllib.loads(p.read_text())["project"]["name"] == name:
+            return member
+    print(f"error: package {name!r} not found in workspace", file=sys.stderr)
+    sys.exit(1)
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser(
+        description="Run pytest for a uv workspace package.",
+    )
+    parser.add_argument("package", help="workspace package name")
+    parser.add_argument(
+        "--exact",
+        action="store_true",
+        help="install only the package's declared deps (CI isolation)",
+    )
+    known, pytest_args = parser.parse_known_args()
+
+    pkg_dir = find_package_dir(known.package)
+
+    cmd = ["uv", "run", "--package", known.package]
+    if known.exact:
+        cmd += ["--exact", "--group", "dev"]
+    cmd += ["pytest", pkg_dir, *pytest_args]
+
+    sys.exit(subprocess.call(cmd))
+
+
+main()